Collagen, a scleroprotein having a molecular weight of about 297.000 Daltons, is the most abundant fibrous protein in the higher vertebrates because it is the principal constituent of the skin, the connective tissue and the organic material present in the bones and the teeth. This protein represents approximately one third of the total amount of proteins in the human body.
Various types of collagen occur naturally and they are all composed of three polypeptides chains which have a constant periodicity and are arranged in a triple helix: the difference between the various types of collagen is due to small differences in the primary structure of the chains, i.e. in the aminoacidic sequence of the chain itself.
Type I collagen, which is the basic constituent of the skin, the bones and the tendons, may be regarded as the most abundant of the various types of collagen; the triple helix has two α1(I) and one α2(I) chain composition where the α1(I) and the α2(I) chains are homologous.
Between the two α1 chains and the α2 chain there are electrostatic interactions, hydrogen bonds and sulfur bond bridges which together with the presence of hydroxyproline confer to the molecule the typical characteristic of spatial rigidity.
The production of collagen in the bodies of mammals is preceded in the cell by the formation of a larger biosynthetic precursor, called procollagen assembled in the triple helix but containing the two non helicized terminal parts; the procollagen is then degraded by specific enzymes which cut off these parts and form collagen.
Out of the cell the molecule assembles itself in polymeric forms called fibrils and fibers: in the tendon this assembling is spatial, while in the skin is planar; fibrils and fibers are the real structure of collagen in the mammals.
The role of collagen in wound healing is well known: platelet aggregation takes place when platelets come in contact with a suspension of collagen fibers (Collagen in health and disease by Barnes M. J., Weiss J. B. and Jayson M.; VI Ed., Churchill Livingstone Ed., Longman Group Ltd., 1982, chapter 10, page. 179) and the protein is fundamentally involved in the mechanism of cicatrization.
The literature discloses the use of collagen as a stimulating agent in the process of wound healing by interaction with various growth factors, for its action of capturing fibronectin, as well as the migration and replication of cells which are the consequence thereof.
Collagen is currently used as a wound-healing agent in clinical surgery, in the treatment of burns, as a vehicle in surgical prosthesis (suture threads, gauzes, etc.) as a material for implantation, or as a component in compositions in the pharmaceutical and cosmetic sector. A known type of compositions containing collagen are spray compositions; a spray is easy to apply to a wound or to skin without contacting them with hands or an applying device. On the other side, it is not very easy to obtain a spray composition that ensures a uniform distribution of the collagen particles on the target surface.
Many studies have demonstrated that collagen having small particle sizes has a number of advantages as a material for skin dressing, and in particular wound dressing, e.g. the fact that small particles have large surface area and that it is easier to prepare spray compositions. Commercially available powder collagens are now mainly in the form of a powder having small particle size in order to improve its adhesion to moist surfaces and to improve its use in the form of a spray.
Small size particles are also seen as a critical feature for collagen in view of the stability of the collagen polymer structure. In fact it was shown that polymer degradation can also be affected by the particle size. For instance, the rate of biomatrix polymer degradation was found to increase with increasing particle size in vitro. It was thought that in smaller particles, degradation products of biomatrix formed can diffuse out of the particles easily while in large particles, degradation products are more likely “trapped” within the polymer matrix for a longer period so as to cause autocatalytic degradation of the polymer material. Therefore, it was hypothesized that larger particles will contribute to faster polymer degradation as well as to a faster decrease of the bioactivity of the product. Clinical aspects will be more influenced from collagen biomatrix preservation; in fact, more stability of the product will be reflected in faster tissue re-epithelialization.
For all the above reasons powdered collagen products that are commercially available preferably have a considerably small particle size.
U.S. Pat. No. 196,185 discloses particulate collagen formed from type I collagen, type III collagen and mixtures thereof, having a particle size from 1 to 50 microns and in particular from 5 to 25 microns. The document further discloses compositions containing such powdered collagen which may be applied as wound dressing. The collagen microparticles of said composition are sufficiently small to be airlessly sprayed through an orifice to form a dry film on the surface of skin or wounds, which promotes wound healing and tissue growth.
WO0160922, in the name of the present applicant, discloses a process for the production of micronized collagen having a particle size of from 5 to 30 microns, generally not more than 20 microns and preferably of approximately 18 microns. This process enables the production of powered collagen which is non-denatured, anallergic, free from impurities or contaminants and in a finely micronized form. The product obtained by this method exhibits good adhesion of the collagen to the wound and, due to its particle size, it may be used in a spray composition.
Thus, according to the prior art, the wound healing properties of powdered collagen are improved by reducing the maximum size of the particles. The preferred particle size according to the prior art is below 20-25 microns.